This invention relates to a method for heat treatment of a glass plate to strengthen a peripheral region of the glass plate by creating plane compressive stresses in a surface of the peripheral region. The heat treatment method is applicable to either a flat glass plate or a curved glass plate.
Glass plates cut into various shapes are widely used in buildings and vehicles, and the use of relatively thin glass plates is increasing. After cutting into desired outline shapes the glass plates are handled for further processing such as laminating or coating, for transportion, for fitting into building or vehicle windows, and so on, and at every stage great care must be taken not to break the glass plates particularly in their edge regions. Therefore, aside from tempered glass plates, there are many proposals of heat treatments for strengthening glass plates only in a peripheral region of each glass plate.
For example, JP No. 32-2684 relates to a sag bending method in which a heated glass plate is supported by a ring-like mold, and proposes to create compressive stresses in a peripheral region of the bent glass plate during annealing of the glass plate on the ring-like mold by making the ring-like mold such that the peripheral region of the glass plate is outside the outer peripheral edge of the ring-like mold and rapidly cooling the protruding peripheral region of the glass plate. However, by this method the annealing process becomes very intricate, and the unsupported peripheral region of the glass plate is susceptible to unwanted deformation such as waving or warping.
JP No. 63-17777 relates to a process of producing a curved and laminated glass panel using two glass plates each having a thickness of 1.5-2.5 mm, and proposes to strengthen a peripheral region of each of two curved glass plates to be laminated while the two glass plates are placed one upon the other on a mold and annealed by rapid cooling of peripheral regions of the both glass plates with blasts of cool air. The rapid cooling is made such that in the temperature range of 450.degree.-550.degree. C. the peripheral region of each glass plate is cooled at a rate of 90.degree. to 150.degree. C./min whereby plane compressive stresses ranging from 250 to 500 kg/cm.sup.2 are created in one surface of the peripheral region of each glass plate. Also in this method the peripheral region of each glass plate is outside the outer periphery of the mold, and therefore there is a possibility of unwanted warping or waving of the peripheral region, and it is not easy to create compressive stresses only in the peripheral region of each glass plate.
JP No. 63-21541 (Utility Model) relates to a glass plate bending process which uses a ring-like mold and includes a tempering step to temper the entire area of the bent glass plate with jets of cool air, and proposes to cover the upper surface of the ring-like mold with a stainless steel fiber cloth. The covering is provided primariry in order that the bent glass plate may not show a significant trace of its contact with the ring-like mold in its peripheral region. Besides, it is expected that the provision of the covering permeable to gases will prevent stagnation of cooling air in a narrow gap between the mold surface and the peripheral region of the glass plate whereby the peripheral region will efficiently be quench tempered as well as the major region of the glass plate. This proposal does not relate to tempering of only a peripheral region of a glass plate.